The technique of shearing interferometry, or shearography involves the interference of two laterally displaced images of the same object to form an interference image. Conventional shearographic methods require that a first interference image (or baseline image) be taken while the object is in an unstressed or first stressed condition, and another interference image be taken while the object is in a second stressed condition. Comparison of these two interference images (preferably by methods of image subtraction) reveals information about the strain concentrations and hence the integrity of the object in a single image called a shearogram. In particular, shearography has been shown to be useful to detect strain concentrations and hence defects in vehicle tires, especially retread vehicle tires.
In conventional electronic shearography, interference images are stored in a computer memory and are compared electronically to produce single static shearograms. Because all the data are processed electronically, the results of the analysis can be viewed in "real time". "Real time", as used in the prior art, refers to the ability to view the shearogram nearly instantaneously after the second interference image has been taken.
An apparatus and method for performing electronic shearography is described in U.S. Pat. No. 4,887,899 issued to Hung. The apparatus described in the cited patent produces an interference image by passing light, reflected from the test object, through a birefringent material and a polarizer. The birefringent material, which can be a calcite crystal splits a light ray, reflected from the object, into two rays, and the polarizer makes it possible for light rays reflected from a pair of points to interfere with each other. Thus, each point on the object generates two rays, and the result is an interference image formed by the optical interference of two laterally displaced images of the same object.
Prior to the developments disclosed in the Hung patent, the spatial frequency of the interference image produced in shearographic analysis was relatively high requiring the use of high resolution photographic film to record a useful interference image. The development disclosed in the Hung patent produces an interference image with a relatively low spatial frequency because the effective angles between the interfering rays are small. Therefore, the interference images can be recorded by a video camera, a video camera normally having much less resolving capability than a high density or high resolution photographic film. By storing an interference image of the object in its initial, unstressed condition, and by comparing that interference image, virtually instantaneously, by computer with another interference image taken under a different level of stress, a "real time" image or shearogram of the resultant strains on the object can be observed. Each point on the actual interference image is generated by the interference of light emanating from a pair of distinct points on the object. Therefore, each pixel of the video camera is illuminated by light reflected from those two points. If the overall illumination remains constant, then any variations in the pixel intensity, in the interference image, will be due only to changes in the phase relationship of the two points of light.
When the initial video image of the interference image is stored, an initial intensity for each pixel is recorded, as described above. If differential deformations occur in the object, such deformations will cause changes in the subsequent interference image. In particular, the intensity of a given pixel will change according to change in the phase relationship between the two rays of light, reflected from the two points on the object, which illuminate the pixel. The phase differences can be either positive changes, causing the pixel to become brighter or negative changes, causing the pixel to become darker. Whether the pixel becomes brighter or darker depends on the initial phase relationship and the direction of the change of phase. Due to the cyclic nature of phase interferences, as the deformation of the object continually increases, the intensity at a given pixel may pass through a complete cycle. That is, the intensity of the pixel might increase to a maximum (positive) difference, then return to the original intensity, and then continue to a maximum (negative) difference, and so on.
In systems of the prior art, a single shearogram is derived from two single static interference images taken at two distinct stress levels. The single shearogram is then viewed by an operator for analysis if multiple shearograms are taken, the analysis is done one shearogram at a time. Thus, the operator attendance time, required to perform a thorough stress analysis, is substantial. Further, a single shearogram may falsely show light features that appear to be defects (referred to as "false positives"). These "false positives" are caused by different reflective characteristics on the surface of the test object and appear as defects when a static shearogram is viewed. Further still, in a static shearogram some real defects may be "washed out" and thus not visible (referred to as "false negatives"), at certain (particularly high) stress levels. These "washed out" effects are caused by shearographic fringe lines that are not spatially separated enough to be visibly distinguishable and therefore appear to be aberrational light effects rather than real defects in the test object. Thus, a single static shearogram may contain inaccurate information with regards to the defects actually present. Furthermore, an operator having to analyze a large number of shearograms requires a large amount of operator attendance time.
There is a need and desire for an improved method of presentation of shearographic images that provide advantages over the prior art. There is also a need and desire for a method of presenting shearographic images that provide improved accuracy, shorter attendance times by an operator, and shorter overall cycle times for a test object. Further, there is a need and desire for a method of presenting shearographic images that reduce the undesirable effects of false negatives by preventing "wash out" of larger defects at high stress levels. Further still, there is a need and desire for a method of presenting shearographic images that allows real defects to be distinguished over light features that otherwise may be confused as defects, thereby minimizing false positives.